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Twenty people were still hospitalized Tuesday, six in critical condition

The explosive setup resembled those used in Iraq and Afghanistan, an official says

Holmes will be formally charged on July 30

The suspect in last week's deadly shooting rampage inside a crowded Colorado movie theater booby-trapped his Aurora apartment with more than 30 homemade grenades and 10 gallons of gasoline, a law enforcement official who viewed video showing the apartment's interior has told CNN.

The setup appears to have been "rigged" to explode, the official said. The forensic analysis is expected to determine whether it would have done so had it not been disabled.

The gasoline was divided into glass containers and gas cans to "enhance the thermal effect" of any explosion, the official said.

Over the weekend, Aurora Police Chief Daniel Oates told reporters there was "evidence of ... some calculation and deliberation" in the Friday attack at the Century Aurora 16 multiplex.

Holmes received deliveries over the past four months at his home and work addresses, which may explain how he may have obtained some of the materials that were used in the attack or found at his apartment, Oates said.

The University of Colorado-Denver School of Medicine, where Holmes enrolled as a doctoral candidate in its neuroscience program but later withdrew, was investigating whether he received any of shipments while working as a research assistant at the Anschutz Medical Campus in Aurora.

Authorities have been tight-lipped about a possible motive in the case; police spokesman Frank Fania told CNN that Holmes has been uncooperative with investigators.

The multiplex, where the shootings occurred during a midnight premiere of "The Dark Knight Rises," will remain shuttered at least until Wednesday while police complete their investigation of the crime scene and allow the suspect's defense team access to it.

On Monday, Holmes made his first court appearance.

Looking dazed at times, Holmes -- who after the attack identified himself to police as "the Joker" -- wore a maroon jumpsuit over a white T-shirt and gave little indication that he was paying attention to the courtroom procedure that ensured he will continue to be held without bond.

"The charges on which the court found cause included first-degree murder," Arapahoe County District Attorney Carol Chambers told reporters outside the courthouse. She said prosecutors have a lot of work to do.

"I would say there's no such thing as a slam-dunk case," she said. "We're still looking at the enormous amount of evidence."

Deciding whether to pursue the death penalty is a long process that involves input from victims and their relatives, she said.

A capital case would require a finding of either extreme indifference or deliberation, she said.

Holmes is being held in connection with the shootings that killed 12 people and left 58 others wounded. As of Tuesday evening, 20 people remained hospitalized -- six of them in critical condition -- in five area hospitals, hospital spokespersons said.

Four of those killed were active members of the the U.S. military. The Department of Defense was flying flags at half-staff in their honor.

Meanwhile, Lisa Damiani, an attorney representing the Holmes family but not the suspect, said his family members were keeping their location secret. "I don't think they would like the media to know where they are," she said, adding that she feared for their safety.

"They're doing as well as they can, under the circumstances," Damiani told reporters at her office in San Diego, California, where the family lives. "I think everyone can imagine how they're feeling -- anyone who's ever been a parent."

She added, "The family has elected not to discuss James or their relationship with James at this time."

The family issued a statement Friday saying, "Our hearts go out to those who were involved in this tragedy and to the families and friends of those involved." It added, "We are still trying to process this information."